Google has specific rules for sitemap generation. It is useful to have your XML sitemaps built automatically with the help of Google XML Sitemap extension for Magento 2 in order to meet all search engines requirements. Faster indexing of new pages shortens the time your new products to appear in Google.

I think it depends on what are you planning to do, if you are planning to exclude some of the URLs then you can't do that out of the box Magento and it doesn't include images as well which is another drawback. You can have a look at point 6 (SITEMAP.XML) and point 8 (SUBMIT IMAGES TO SEARCH ENGINES)